1. Mongodb Operators Cheat Sheet Example
2. Mongodb Query Language Cheat Sheet

MongoDB is a popular NoSQL database that allows unauthenticated access by default.

About this Cheat Sheet Basic Information The idea behind this is to have all (well, most) information from the above mentioned Tutorial immediately available in a very compact format.

1. MongoDB Cheat Sheet Db Info Commands Start a mongo shell to remote Db Commands Users Create User Backup / Restore Backup Restore with mongorestore Find Querying References Db Info Commands Start a mongo shell to remote Db mongo -host Test01 -port -username adminuser -password -authenticationDatabase admin.
2. MongoDB Cheat Sheet Show All Databases Show Current Database Create Or Switch Database Drop Create Collection Show Collections Insert Row Insert Multiple Rows Get All Rows Get All Rows Formatted Find Rows Sort Rows Count Rows Limit Rows Chaining Foreach Find One Row Find Specific Fields Update Row.
3. The MongoDB database query operator cheat sheet is an excellent resource for beginners and experts alike. It contains comparison, array, logical, evaluation, element and more commonly used query operators. Refer to it often. Think of the cheat sheet as your helpmate for efficient coding in all of your MongoDB projects.
4. Performance tuning is not trivial, but you can go a long way with a few basic guidelines. In this blog, we will discuss how you analyze the workload of your MongoDB production systems, and then we’ll review a list of important configuration parameters that can help you improve performance.

Regardless of the user’s authentication database, Mongo always stores user information in admin.

MongoDB stores all user information, including name, password, and the user’s authentication database, in the system.users collection in the admin database.

See centralized-user-data and system-users-collection.

When you create a user and grant that user access to a single database (aka their authentication database) then that information can only be stored in the admin database.

So, it’s not really a question of “best practice”; storing user details in admin is MongoDB’s choice, as implemented by their user management commands.

Update in response to this comment:

Ok, so the users are always located in the admin db, but I may also add “duplicates” to the other dbs? Maybe the question should be whether there any advantage in adding users to the other “non admin” dbs?

If you intend to have a single user with access to multiple databases then create a single user with roles in each of those databases rather than creating that user multiple times i.e. once in each of those databases. For example:

Mongodb Operators Cheat Sheet Example

Create initial admin user

Sharded Cluster with enforced authentication

Create:

• a cluster-wide admin user
• a replica set specific admin user

Cluster-wide Admin

Replica Set Admin (a.k.a shard local)

Enable authentication in the mongos configuration

Connect to all replica set member nodes

Authenticate and check that the admin users exist

Log in

Rename collections

Copy Collection

Check replication set status

Backup/Restore

Documents

Admin commands

TLS 1.2 for Mongo Routers

To protect your application’s database connection enable TLS on the mongo routers as follows. Note that your mongo driver configuration needs to trust the CA certificate and enable transport encryption with ssl=true.

Rolling Update/Cluster Patching

Maintenance (startup in reverse order):

Replication Concept

1. write operations go to the primary node
2. all changes are recorded into operations log
3. asynchronous replication to secondary
4. secondaries copy the primary oplog
5. secondary can use sync source secondary*

• automatic failover on primary failure

*settings.chainingAllowed (true by default)

Replica set oplog

• special capped collection that keeps a rolling record of all operations that modify the data stored in the databases
• idempotent

• default oplog size (for Unix and Windows systems):

  Storage Engine	Default Oplog Size	Lower Bound	Upper Bound
  In-memory	5% of physical memory	50MB	50GB
  WiredTiger	5% of free disk space	990MB	50GB
  MMAPv1	5% of free disk space	990MB	50GB

Deployment

• start each server with config options for replSet
  /usr/bin/mongod --replSet 'myRepl'
• initiate the replica set on one node - rs.initialize()
• verify the configuration - rs.conf()
• add the rest of the nodes - rs.add() on the primary node
  rs.add('node2:27017') , rs.add('node3:27017')
• check the status of the replica set - rs.status()

Sharding

Components

• shard/replica set - subset of the sharded data
• config servers - metadata and config settings
• mongos - query router, cluster interface
  sh.addShard('shardName')

Shards

• contains subset of sharded data
• replica set for redundancy and HA with odd number of voting members
• primary shard
• don’t shard collections if dataset fits into single server
• –shardsvr in config file (port 27018)
• every xxx has a primary shard per database
• all non-shared collections will reside on primary shard

Shard keys (and limitations)

• shard keys are immutable with max size of 512 bytes (can not be updated/changed)
• must be ascending indexed key or indexed compound keys that exists in every document in the collection
• cannot be multikey index, a text index or a geospatial index
• update operations that affect a single document must include the shard key or the _id field
• no option for sharding if unique indexes on other fields exist
• no option for second unique index if the shard key is unique index
• ranged sharding may not distribute the data evenly
• hashed sharding distributes the data randomly

Config servers

• config servers as replica set (only 3.4)
• stores the metadata for sharded cluster in config database
• authentication configuration information in admin database
• holds balancer on Primary node (>= 3.4)
• –configsvr in config file (port 27019)

mongos

• caching metadata from config servers
• routes queries to shards
• no persistent state
• updates cache on metadata changes
• holds balancer (mongodb <= 3.2)
• mongos version 3.4 can not connect to earlier mongod version

Sharding collection

Mongodb Query Language Cheat Sheet